Substrates made of various hard and brittle materials such as silicon wafers and glass panels are subjected to a polishing process using loose abrasive grains to achieve a high degree of flatness and a low surface roughness so as to obtain two surfaces completely in parallel with each other and to allow the entire surfaces to be used. Specifically, in a state where a slurry containing abrasive grains is held by cells opened on a polishing surface of a polishing pad, the polishing pad and a substrate are rubbed against each other, and thus the surface of the substrate to be polished is processed into a flat surface with the abrasive grains in the slurry. In the polishing with loose abrasive grains, a tendency is observed in which the temperature of the substrate is raised from moment to moment by the frictional heat during the polishing, and the chemical action of the slurry is activated, so that the polishing rate increases. In addition, local heat generation also tends to occur, which causes unevenness in polishing rate within a surface of a substrate, resulting in a problem of deterioration in the degree of flatness. Especially, wafers for LSI production have been gradually increasing in size to improve the productivity nowadays. Recently, wafers having diameters of 200 mm or more have been commonly used, and even wafers having diameters of 300 mm or more have also been produced. With such an increase in size of wafers, the polishing rate tends to be different between a center portion of a wafer and an outer circumferential portion of the wafer, making it extremely difficult to polish a wafer surface uniformly.
To improve the flatness and the surface roughness of a substrate, various polishing pads have been proposed. For example, Japanese Patent Application Publication No. 2012-714 (Patent Literature 1) discloses a polishing pad in which a polishing layer provided on a surface and configured to be brought into pressure contact with a polishing workpiece is formed of a foamed polyurethane including multiple cells, and in which 90% or more of the multiple cells are cells formed such that the inner spaces of the cells are spatially separated from one another.
However, in the case of a polishing pad substantially consisting of closed cells as described above, portions holding the slurry are limited to only portions opened on the surface, and hence the amount of the slurry held is small, so that a cooling effect cannot be obtained sufficiently by the supplied slurry. Presumably because of this, the polishing temperature distribution is uneven, and the polishing is promoted in portions where the polishing temperature is high, so that the surface to be polished cannot be polished uniformly. This trend becomes more pronounced with increase in polishing time, and the degree of flatness deteriorates with time. Moreover, since clogging with foreign substances such as polishing dust proceeds rapidly in the above-described polishing pad, the amount of the slurry held is further reduced, and the slurry held is localized. This causes such a problem that the resultant substrate has a tapered shape or such a problem that excessive polishing of the outer circumference of the substrate causes the edge roll-off. Even when the foreign substances such as polishing dust are removed by dressing with a dresser, the same problems occur soon after several times of polishing.
In addition, Japanese Patent Application Publication No. 2011-20234 (Patent Literature 2) discloses a polishing pad in which a polishing layer configured to be brought into pressure contact with a polishing workpiece is made of a foam having closed cells and open cells, wherein the closed cell ratio, which is the ratio of the volume of the closed cells relative to the total volume of the cells including the closed cells and the open cells, is 0.3 to 10%.
However, since most of the cells in the above-described polishing pad are connected with each other, the resin easily deforms upon reception of a polishing load. Hence, although the formation of scratches on the polishing workpiece can be suppressed to some degree, the requirement for a higher degree of flatness cannot be satisfied.